I. Field of the Invention
This invention relates generally to fluid pumps, and more particularly to an improved piston-type positive displacement pump in which the stroke length of the piston or pistons utilized in the pump can be manually adjusted to vary the pump output for a given drive speed.
II. Discussion of the Prior Art
In certain fluid handling applications, it is often desirable to introduce a precise, predetermined quantity of a chemical into a fluid stream so that the chemical will be dispensed in a desired ratio with a carrier fluid. For example, in agricultural spraying applications, liquid herbicides or pesticides may be introduced into a stream of water before the water is dispensed through a spray boom onto the field. In another unrelated application, that of fighting fires, if a small quantity of a chemical surfactant is introduced into the high pressure water stream, a foam is created which is particularly effective in extinguishing certain types of fires.
It is known in the art that a positive displacement pump can be used to effectively meter into a carrier stream a precise, measured quantity of a chemical additive. A typical positive displacement pump will include one or more pistons which are arranged to move in reciprocating fashion within a cylindrical chamber by means of a conventional crankshaft and connecting rod assemblage. On each suction stroke of a piston, a predetermined quantity of the chemical additive will be drawn into its cylindrical chamber and, during the pressure stroke, that quantity will be ejected into the carrier stream. The flow rate of the chemical additive is, of course, a function of the speed at which the pump's shaft is driven as well as the stroke volume of each piston.
In order for a given pump to be able to dispense precise quantities of a chemical into a fluid stream, it is also desirable to be able to manually adjust the piston's stroke. It is desirable that this be done without having to disassemble the pump to alter the eccentric or to substitute pistons and/or connecting rods of differing dimensions.
The John Blue Company of Huntsville, Ala., has manufactured and sold a variable stroke, positive displacement pump in which the eccentricity of the pump's crankshaft can be adjusted by rotating an arm from a location exterior to the pump's crankcase. The mechanical design whereby this is accomplished is set out in a publication entitled "Instruction Manual N-Serve Pump & Carriage Assembly" available from the John Blue Company and those desiring more information on that specific implementation is referred to that publication. Suffice it to say, this prior art pump and especially the stroke adjustment mechanism used therein, is relatively complex in terms of the number of parts required. Moreover, to adjust the stroke length, it is first necessary to remove a locking screw and nut combination which normally holds the stroke-setting arm fixed relative to a toothed-flange which is keyed to a spacer member and which, in turn, is coupled to eccentric rings disposed between the pump's shaft eccentrics and its connecting rods. Once the locking screw is removed, the stroke-setting arm can be turned independent of the flange and with it the eccentric rings. Once the locking screw is loosened, the earlier setting is disturbed and lost. Then when the arm is repositioned at a new setting, it is difficult to retain that setting while again reassembling the locking screw and nut combination.